Many battery technologies have difficulty when operating at very low temperatures. Of particular concern are applications such as cold weather engine starting. In such conditions the ability of the battery system to deliver high cranking amps at very cold temperatures is crucial but inherently limited in chemical batteries.
A number of inventions address the fundamental issue of cold batteries by teaching a variety of ancillary heating systems such as those that are wrapped around the battery like a blanket or placed below the battery like a warming plate. These ancillary heating systems often rely upon an external power supply, such as household AC. Thermostatically controlled battery heaters of this type are well known. However, these types of battery heaters have a number of disadvantages when applied to gelled or solid electrolyte systems such as Lithium Polymer batteries. For example, solid electrolyte batteries cannot efficiently absorb the heat applied from an external source as efficiently as a liquid electrolyte battery can. Furthermore, external heating elements placed against the casing of a solid electrolyte batter can cause thermal stress and damage to the battery. External heating elements and their electrical connections are also very fragile and so are a major source of failure in battery heating systems.
A number of solutions have been proposed to overcome deficiencies of external battery heating systems.
One example is U.S. Pat. No. 7,327,122 ‘Battery Heating Circuit’ issued to Kamenoff on Feb. 5, 2008 teaches a heating system that is powered by the battery itself. The battery heater is powered automatically when the battery is subjected to an external load. However, in most situations the battery needs to be warm before the load is applied so that the load can be properly supplied. In addition, the battery heater draws power from the battery which may further compromise the performance of the battery when supplying a large load in cold temperatures. For a lithium ion battery, if the battery has insufficient available capacity at low temperatures to support a load, then the terminal voltage of the battery will drop and this can severely damage the battery. A lithium cobalt battery normally operates at between 3.0 and 4.2 volts. If the terminal voltage of the cell is brought below about 2.0 volts, irreversible damage to the battery will occur.
A further example is U.S. Pat. No. 6,441,588 ‘Battery charging control method employing pulsed charging and discharging operation for heating low-temperature battery’ issued to Yagi et al on Aug. 27, 2002. Yagi proposes a system that alternately applies an external load to a cold battery followed by an external charging pulse. This alternate loading and charging cycle heats the battery. The Yagi system is useful when a battery needs to be charged rapidly in cold temperatures. However, this system is not effective when a battery must be discharged in cold temperatures such as in an engine cold start. Externally loading the battery to heat it results in wasted energy.
Another example is U.S. Pat. No. 6,882,061 ‘Battery self-warming mechanism using the inverter and the battery main disconnect circuitry’ issued to Ashianti et al on Apr. 19, 2005. Ashianti teaches the use of a split battery bank and motor driver to effectively convert DC to AC to DC as a means for warming the battery. However, this system is too cumbersome to implement inside the battery casing and must be mounted externally.
A further example is U.S. Pat. No. 6,072,301 ‘Efficient resonant self-heating battery electric circuit’ issued to Ashianti et al on Jun. 6, 2000. This patent teaches a four step process: transferring energy from the battery to an inductor; then to a capacitor; then back to the inductor; then back to the battery. This is a complicated process which requires additional components which are not well suited for installation within the battery casing. Energy is transferred by a single switch to an energy storage element, and is then transferred back into the battery through the same switch. The body of the patent discloses a system based on the resonant characteristics of an inductor and capacitor being operated in a tuned fashion at very high power. The four step energy transfer process will suffer from poor efficiency, especially at high power levels. Large batteries have very low impedance, even when they are cold, the result is that most of the heat generated by this system will be lost in the inductor and capacitor, rather than inside the battery cells, which is where it is needed. The capacitor will also suffer from severe degradation at low temperatures and will itself need to be heated in order to work efficiently in this system.
A resonant system based on a combination of capacitors and inductors will also require more components than necessary. The use of two separate energy storage elements, a capacitor and inductor, will add significant weight and size to the end product and may preclude the warming circuit being enclosed within the battery housing itself.
There exists a need for a battery self warming circuit that requires fewer or smaller energy storage elements that can be operated in a manner that ensures maximum heat generation inside the battery cells, rather than inside the self-heater driving circuitry.
Prior art resonant systems rely upon the operation of energy transfer circuits that are connected to the battery. These circuits used are resonant circuits composed of inductors and capacitors having a fundamental frequency defined by the circuit elements themselves. There remains a need for a battery heating system that can efficiently heat a battery without an external energy source.
There also a need for a battery heating system that can use the battery's own energy without damaging the battery by overloading it at cold temperatures. There is also a need for a battery heating system that does not use a discrete heater element.
There is a need for a resonant circuit that is governed by the state of the battery itself, the individual cells and the current and voltage profile of those cells when subjected to a load.